1. Field of the Invention
The present invention relates to a hydraulic control apparatus for an automatic transmission of a motor vehicle.
2. Discussion of the Prior Art
There is known a hydraulic control apparatus for an automatic transmission of a motor vehicle having a plurality of hydraulically operated frictional coupling devices which are selectively activated for selectively establishing a first-speed position and at least one higher-gear position whose speed reduction ratio is lower than that of said first-speed position. This hydraulic control apparatus includes a pressure regulating valve for regulating a line pressure of a working fluid to be applied to the hydraulically operated frictional coupling devices, and a cut-back valve which has a first position and a second position. The cut-back valve is placed in the first position when the transmission is placed in the first-speed position, and is placed in the second position when the transmission is placed in any higher-gear position. In the first position, the cut-back valve causes the pressure regulating valve to raise the line pressure by a predetermined amount to a higher level from a normal level as normally established by the pressure regulating valve. In the second position, the cut-back valve causes the pressure regulating valve to regulate the line pressure to the normal level. An example of such hydraulic control apparatus is disclosed on pages 73-75 of "Fundamentals of AUTOMATIC TRANSMISSIONS, Service Training Information" published as Pub. No. ST1009E in 1988 by the assignee of the present application, and on pages 4-50 and 4-51 of "TOYOTA CORONA, New Car Features" published in August, 1985 by the assignee.
When a motor vehicle equipped with such automatic transmission is started on a road surface having a low coefficient of friction such as a snow-covered or frozen road surface, it is desirable to start the vehicle with the automatic transmission placed in a higher-gear position such as the second-speed position whose speed reduction ratio is next lower than that of the first-speed position, so as to reduce the tendency of slipping of the drive wheels on the road surface. If the automatic transmission is controlled by the hydraulic control apparatus of the type described above, however, the line pressure generated by the pressure regulating valve is not increased upon starting of the vehicle, since the second-speed position is selected. In this case, therefore, the torque transmitting capacity of the frictional coupling devices is insufficient for starting the vehicle with a sufficient drive force transmitted to the drive wheels.
Although the above drawback may be solved by using the frictional coupling devices which torque transmitting capacity is increased by increasing number of friction members of the coupling devices, this solution unfavorably results in increasing the size of the transmission.
In view of the above drawback, the present inventors developed a hydraulic control apparatus which has mode selecting means for selecting a high-gear start mode in which the vehicle is started with the automatic transmission placed in a predetermined one higher-gear position such as the second-speed position, and pressure applying means, responsive to the mode selecting means, for applying to the cut-back valve a first and a second pressure to be applied to respective two coupling devices of the transmission to establish the predetermined one higher-gear position, for placing the cut-back valve in the first position to raise the line pressure to the higher level when the transmission is shifted to the predetermined higher-gear position upon starting of the vehicle in the high-gear start mode. For instance, the high-gear start mode is established when the vehicle is started with the shift lever placed in a second-range position (S), rather than in a drive-range position (D) and when a suitable high-gear start switch is turned on. The number of the forward drive gear positions available with the shift lever in the second-range position is smaller than that available with the shift lever in the drive-range position.
The above hydraulic control apparatus assures sufficient torque transmitting capacity of the frictional coupling devices when the vehicle is started in the high-gear start mode. However, the above arrangement causes the line pressure to be raised to the higher level even when the transmission is shifted up to the higher-gear position (e.g., second-speed position) while the shift lever is placed in the second-range position, but while the high-gear start switch is off. In this case, therefore, the transmission suffers from a shifting shock upon engagement of the appropriate frictional coupling devices with relatively high hydraulic pressure due to the rise of the line pressure to the higher level, while the vehicle is running with the shift lever placed in the second-range position.
To solve the above problem, it is required to control the back pressures of the appropriate frictional coupling devices, in order to smoothly raise the hydraulic pressure for engagement of the coupling devices, when the transmission is shifted up to the higher-gear position while the vehicle is running with the shift lever placed in the second-range mode, and with the high-gear start switch in the off position. This solution, however, results in increasing the cost of the apparatus, because it requires suitable means for controlling the accumulator back pressures.